Until the early 1950's, commonly used pressure-sensitive adhesives for skin applications were based on natural or synthetic rubber compositions compounded with low molecular weight tackifiers, plasticizer, stabilizers, etc. These adhesives had the disadvantage of being hydrophobic and incapable of absorbing water. Thus, such adhesives would trap water under the covered area, often causing skin maceration or other skin damage. Furthermore, the low molecular weight substances compounded into these adhesives often would penetrate the skin, causing irritation or sensitization.
Polyacrylate pressure-sensitive adhesives are an improvement over the rubber-based adhesives, partly due to their self-adhesive property. This property allows them to be prepared as single-component polymeric materials without the need for potentially allergenic modifying or tackifying agents. However, these adhesives often contain unreacted residual acrylic monomer as an impurity in an amount which would irritate and/or sensitize skin. Although these polyacrylate pressure-sensitive adhesives are much more permeable to moisture or water vapor than are the rubber-based adhesives, they are incapable of absorbing any significant amounts of moisture or water. Therefore, when used for long duration in skin or wound care applications, adhesion is compromised and/or skin damage or maceration may result.
Polyurethanes are polymeric products of diols or polyols and diisocyanates or polyisocyanates. Despite the broad applications of polyurethane chemistry, polyurethane based pressure-sensitive adhesives are not widely used and to date have been found suitable for only a few specialized applications. A suitable balance of elastic and viscous properties which is required in pressure-sensitive adhesives has not been readily attainable in conventional polyurethane materials.
Existing polyurethane based adhesives are typically comprised of aromatic polyisocyanates which function either as weak elastomers or simply as high viscosity liquids. The adhesives composed of the elastic type polyurethanes tend to fail by gradually peeling away from surfaces to which they have been applied. The high viscosity type polyurethanes, which are typically obtained by using a substantial excess of polyol, leave a residue upon removal, and their cohesive strength is too low to withstand the stresses applied in many applications. The difficulty of attaining this balance of viscoelastic characteristics in a polyurethane explains the limited effective use of polyurethane pressure-sensitive adhesives for medical devices applied to the skin.
An advance in pressure-sensitive adhesive formulations for skin and particularly for wound care applications was the development of compositions comprising blends of one or more water-soluble or swellable hydrocolloids and a tacky, viscous, polymeric material such as polyisobutylene as disclosed in Chen U.S. Pat. No. 3,339,546. Another example is Doyle et al. U.S. Pat. No. 4,551,490 which discloses medicinal grade pressure-sensitive compositions containing polyisobutylenes or blends of polyisobutylenes and butyl rubber, a styrenic radical or block type copolymer, mineral oil and water soluble hydrocolloid gum and a tackifier. Such hydrocolloid containing pressure-sensitive adhesives have the advantage of providing the desired adhesion to skin and, at the same time, are capable of absorbing transepidermal water loss (i.e., perspiration) or other body fluids, including wound exudates.
Existing hydrocolloid-containing pressure-sensitive adhesives have found use in medical applications such as ostomy devices and wound dressings, where the adhesives maintain the device on skin for several days without skin damage. However, existing hydrocolloid-containing pressure-sensitive adhesives have certain limitations in that they lack quick initial tack and tend to disintegrate upon excessive water absorption. Furthermore, existing hydrocolloid-containing pressure-sensitive adhesives are not flexible and/or easily conformable or repositionable on the skin. In addition, they often leave an undesirable residue on the skin.
Another approach to pressure-sensitive adhesives, useful in medical applications, was the development of hydrophilic adhesives. A hydrophilic adhesive which is a variation of polyacrylate based pressure-sensitive adhesives are disclosed in U.S. Pat. Nos. 4,914,173 and 5,017,625 to Ansell. The specific pressure-sensitive adhesives disclosed therein are obtained by reacting an isocyanate prepolymer, which is the reaction product of a poly-functional isocyanate and a polyoxyalkylene diol monoalkyl ether, with a hydroxy-containing ester of acrylic or methacrylic acid to form a functionalized prepolymer and then cross-linking the polymer by irradiation to form a pressure-sensitive adhesive that is not self-adherent but is capable of absorbing (hydrophilic) up to 95% by weight of water when hydrated. Such adhesives have a relatively low concentration of urethane groups and therefore do not have sufficient tack or initial adhesive properties to be adherent to the skin for certain uses.
Despite these efforts, commercially acceptable pressure-sensitive adhesives for use particularly with medical devices which attach to the patients' skin such as wound dressings and ostomy appliances have not been successful. While polyurethane based adhesives generally are less irritating to the skin than acrylic based adhesives and have better strength, polyurethane based adhesives remain problematical. This is because it has heretofore been difficult to provide a polyurethane based adhesive which exhibits excellent adhesion over an extended period of time (e.g. 5 days) while maintaining a high degree of cohesive strength so that the medical device can withstand the rigors of 5 day wear. In addition, prior methods of preparing polyurethane based adhesives are disadvantageous because it is difficult to obtain a consistently uniform product. Such methods typically employ reactions conducted at elevated temperatures. The reactants and the conditions under which they react (e.g. elevated temperatures) lead to variations in product quality and consistency.
It would therefore be a significant advance in the art of medical devices applied to the skin to provide an adhesive composition which exhibits excellent adhesion and excellent cohesive strength over extended periods of wear without irritating the patients' skin. It would be a further advance in the art to provide methods of forming pressure-sensitive adhesives especially suitable for medical applications which consistently provide a high quality product and more consistency and quality in the final product.